Principales principios contables

In the previous sections on field based research and on environmental chamber studies, it has been highlighted the role of air movement on thermal comfort. In warm and hot environments, air movement usually has a positive effect on thermal comfort. On the other hand, it is a cause of discomfort in cool and cold conditions, where the concept of draught applies. This section aims to analyse how the heat is exchanged between

a human body and the surrounding thermal environment, and to explain therefore the biophysical principles of fan use and its impact on the users.

The heat exchange between a human body and the surrounding thermal environ- ment has been extensively investigated (Fanger, 1970; Gagge et al., 1967; ASHRAE, 2013b). It occurs through four mechanisms, namely conduction, convection, radia- tion, and evaporation (Figure 2.7). Conduction refers only to solids, and therefore this mechanism is not affected by the use of a fan, or in general by variation of the air speed. A human body may exchange heat with its surroundings by conduction only when there is a direct contact with a surface at different temperature. Similarly, ra- diation heat exchanges occurs due to the temperature difference between the surface of the human body and the surrounding surfaces, and they are therefore not affected by air speed variations on the human body.

Figure 2.7: Thermal Interaction of Human Body and Environment (ASHRAE, 2013b).

Air movement affects the convective and evaporative heat loss from the skin. The convective heat losses from the outer surface of a clothed body can be expressed as a function of the clothing area factor (fcl), a convective heat transfer coefficient (hc)

evaluated at the clothing surface, and the difference between the mean temperature of the outer surface of the clothed body (Tcl) and the air temperature (ASHRAE,

2013b):

C = fclhc(Tcl− Ta) (2.1)

When a fan is used to increase the air speed on the outer layer (this might be the skin or clothed areas) of a human body, the convective heat transfer coefficient varies. In particular, the more elevated the air speed is, the higher this coefficient becomes. Thus, providing that the temperature difference in equation 2.1 does not change and it is positive (meaning that air temperature is lower than tcl), then convective heat

losses increases, too. On the other hand, if this temperature difference was negative, then the direction of the heat flow would be the opposite. In hot conditions, this may set a temperature limit for the usability of air movement to provide cooling, and the thresholds discussed in the previous sections support this idea. The highest air temperature limit, 34◦_{C at 50% relative humidity, was reported by Huang et al.}

(2013). On the other hands, convection is not the only heat loss mechanism affected by air movement, and therefore those limits might depend also on other things.

Evaporative heat losses from skin depend on the amount of moisture on the skin (expressed by the skin wettedness w), the difference between the water vapour pressure at the skin (psk,s) and in the ambient environment (pa), evaporative heat transfer

resistance of clothing layer (Re,cl), and the evaporative heat transfer coefficient (he)

(ASHRAE, 2013b):

Esk =

w(psk,s− pa)

Re,cl+ 1/(fclhe)

(2.2) Skin wettedness is defined as “the fraction of the skin that is covered with water to account for the observed total evaporation rate” Gagge (1937). Thus, the highest achievable rate of evaporation occurs when skin wettedness is equal to one. Evapora- tive heat loss is a combination of two components: the evaporation of sweat due to thermoregulatory control responses, and natural diffusion of water through the skin.

Without sweating, skin wettedness caused by diffusion is approximately 0.06 for nor- mal conditions, but this figure may decrease down to 0.02 in very dry environments (ASHRAE, 2013b). With sweating, the human body regulates the sweat rate, and the skin wettedness then varies accordingly.

When a fan is used to increase the air speed on the outer layer of a human body, air at certain relative humidity, and therefore water vapour pressure, replaces the air near this outer layer. Evaporation would occur also without this air movement, providing that water vapour pressure difference in equation 2.2 is positive, but at a lower rate. The forced convection generated by the fan increases the rate of evaporation, and this has a noticeable importance in presence of sweat. The sweat production remains the same, but the sweat evaporate faster, and thus the skin wettedness is lower. This is important since skin wettedness is strongly correlated with warm discomfort and is also a good measure of thermal stress ASHRAE (2013b). In very humid environments, the effect of air speed on evaporative heat losses is lower as the air moved by the fan on the outer layer is already close to saturation.

Considering both convective and evaporative heat exchanges, it is therefore pos- sible to say that, in general, the cooling effect of a fan is higher in dry conditions and when the air temperature is lower, and then it decreases as ambient relative humidity and air temperature decreases. Furthermore, there are other factors that may affect the air speed preferences of people since the human perception of air movement in- volves also non-thermal sensory perception of air motion through mechanoreceptors in the skin, particularly near hair follicles de Dear et al. (2013). Taking into account all these elements, plus the fact that there might be significant difference between different human bodies, it appears important to highlight that limits (temperature and relative humidity) for the use of air movement as a cooling means may vary according to the group of people considered. Nevertheless, the biophysical principles discusses in this section remain valid.